One type of memory is a dynamic random access memory (DRAM). A DRAM includes memory cells arranged in rows and columns in an array, with the rows extending along an x-direction and the columns extending along a y-direction. Conductive word lines extend across the array of memory cells along the x-direction and conductive bit lines extend across the array of memory cells along the y-direction. A memory cell is located at each cross point of a word line and a bit line. Memory cells are accessed using a row address and a column address.
DRAM memory cells include a capacitor and an access device, such as a transistor, for accessing the capacitor. Data is stored in the capacitors of DRAM memory cells in the form of electric charges. Data retention time is therefore limited, since over time the stored charge on each capacitor gradually leaks off. To prevent data corruption, the charge on each capacitor is periodically refreshed. The time within which a refresh is performed to prevent such data corruption is commonly referred to as the refresh interval. To refresh data in a memory array, the array is typically placed in a sense mode that senses the data stored in a row of memory cells and writes the data back to the memory cells, thus maintaining the stored data.
DRAM memory cells are typically refreshed in response to auto refresh commands issued by a memory controller in a host. Each auto refresh command refreshes memory cells along a specified number of word lines. The memory controller issues the refresh commands such that the memory cells along all the word lines within the memory array are refreshed within a specified period, such as 32 ms. Extending the refresh interval (i.e., reducing the refresh frequency) consumes less power, which is desirable to reduce power consumption. If the refresh interval is extended beyond the data retention time, however, errors can occur resulting in reduced data quality. Shortening the refresh interval (i.e., increasing the refresh frequency) improves the data quality, but can result in excess power consumption.
The data retention time of a DRAM decreases as the temperature of the DRAM increases. Typical DRAMs, which have a fixed refresh interval, can only be operated at temperatures up to 100° C. without increasing the refresh frequency (i.e., increasing the frequency of the auto refresh commands) to avoid data loss. Future applications for DRAM, such as graphic cards, however, may develop temperatures greater than 100° C., such as 120° C.
For these and other reasons, there is a need for the present invention.